HTTP 401 codes are returned by web pages that require authorization. This shouldn't be confused with pages that require a traditional login.
What causes a 401 error?
A HTTP status code is returned with every request from a server. When the server responds to any request over the web it returns a HTTP status code, usually '200' to say everything is good. If the server returns a 401 code, it means that the server was expecting the browser to send some authorization headers, but it didn't get any.
Realistically there are a few reasons this could actually happen. The first is a misconfigured server or backend code, that is sending bad requests. This happens occasionally, especially with more complex systems. Alternatively you could be linking to an asset or page that does actually require authorization, which is something you probably don't want to do. Another possible explanation is that your page has a link to an API that requires header-based authorization headers; you probably don't want to be linking to these either.
How does Janitor check for a 401?
Janitor crawls your website like a user would navigate or a search engine would crawl; by visiting your most popular and prudent links on each page as it finds them. Any that result in a 401 will be logged, and you'll be notified at the end of the crawl.
We'll let you know the page that returns the 401 and the page that linked to it, so the problem should be easy to fix.